1. Field of the Invention
The present invention relates to light-emitting elements that employ electroluminescence. In addition, the present invention relates to light-emitting devices and electronic devices having such light-emitting elements.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a substance having a light-emitting property is interposed between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is a self-luminous type, there are advantages such as higher visibility of a pixel than visibility of a liquid crystal display, and unnecessity of a backlight. Accordingly, such a light-emitting element is considered to be suitable as a flat panel display element. In addition, other advantages of such a light-emitting element are that the element can be manufactured to be thin and lightweight and the response speed is very high.
Since the light-emitting element can be formed into a film shape, surface light emission can be easily obtained by forming a large-area element. This is a feature that is difficult to be obtained by point sources typified by a filament lamp and an LED or linear sources typified by a fluorescent light. Therefore, utility value as a surface light source that can be applied to lighting devices or the like is also high.
Light-emitting elements using electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
When an organic compound is used as a light-emitting substance, electrons and holes are injected into a layer including a light-emitting organic compound from a pair of electrodes by voltage application to a light-emitting element, so that a current flows therethrough. The electrons and holes (i.e., carriers) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element.
It is to be noted that the excited state generated by an organic compound can be types of a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In an attempt to improve performance of such a light-emitting element, there are many problems which depend on the material, and in order to solve these problems, improvement of element structure, development of a material, and the like have been carried out.
For example, in Reference 1 (Tetsuo Tsutsui and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)), a hole-blocking layer is provided so that a light-emitting element using a phosphorescent material efficiently emits light.